1. Field of the Invention
This invention relates to the manufacture of thin homogeneous flake or powder material from inorganic salt, metallic or similar materials or mixtures of these materials, and more particularly to a manufacturing method and equipment for the continuous production of electrolyte material used in Carbonate Fuel Cell (“CFC”) power plants.
2. Description of the Related Art
A fuel cell is a device that directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. In general, a fuel cell includes a negative or anode electrode and a positive or cathode electrode separated by an electrolyte matrix, which serves to conduct electrically charged ions between the oppositely charged electrodes. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive layer in between each cell.
The electrolyte matrix of a fuel-cell is usually in the form of an electrolyte-impregnated matrix structure. Methods for fabricating such structures are described in, for example, U.S. Pat. Nos. 3,120,456; 3,351,491; 4,009,321; 4,079,171; 4,216,278; 4,591,538; and 5,468,573; Japanese Patent Nos. JP726833, JP09027332, and JP07226513; and European Patent No. EP0689258A1.
In current CFC technology, the electrolyte matrix includes a porous ceramic support impregnated with a molten eutectic electrolyte. Impregnation of the ceramic matrix with electrolyte typically occurs in-situ during first time heat-up and conditioning of the fuel cell stack. The active electrolyte presently used is a melted eutectic mixture of one or more of the inorganic salts lithium carbonate (Li2CO3), potassium carbonate (K2CO3), and sodium carbonate (Na2CO3). Secondary electrolyte additives such as the carbonates of alkaline earth elements magnesium, calcium, barium and strontium, such as, for example, magnesium carbonate (MgCO3) or calcium carbonate (CaCO3), or the oxides of such alkaline earth elements, such as magnesium oxide (MgO), or combinations thereof, may also be added if desired to reduce the cathode dissolution in the liquid electrolyte.
In the present state of the art, a fuel cell stack may include several hundred cells which are stacked in series, each separated by an electrolyte-filled matrix component. CFC demonstration units of several hundred kilowatts in size are currently in operation, and commercial unit market entry is planned for shortly. Electrolyte material production processes therefore must be established to meet the required demands of this commercialization effort. Electrolyte must also be fabricated at a cost that enables the CFC to be commercially competitive within the electric power generation markets.
This presents several significant challenges in light of the electrolyte requirements. In order for the CFC to work properly, the carbonate electrolyte used must be of a precise, uniform composition to form a low melting point eutectic mix. Stringent requirements on the physical characteristics of the electrolyte powder (e.g., particle size, morphology, and angle of repose) are also imposed which must be carefully controlled to enable precise loading into the CFC electrodes and cell hardware using existing continuous production methods and equipment.
Methods currently employed for manufacturing CFC electrolyte typically include weighing a precise amount of electrolyte precursor powders followed by mixing in a batch process using a P-K type blender, ball mill or other similar device. The homogeneous powder mix is transferred to one or more high-temperature crucibles and heated to above the melting point of the mixture of electrolyte precursor powders to form a molten eutectic mix. The molten mix is frozen in the crucible as a slug or is frozen into large, non-uniform chips by dumping the molten liquid into metal pans and allowing it to cool, a process known as “splat-cooling.” While the chips formed by the latter method are more desirable, splat-cooling presents considerable hazard and risk to equipment operators by exposing them to molten liquid splatter. Regardless of method used, the solidified electrolyte slug or chips must then be broken into smaller pieces and finally transferred to a pulverizing device such as a hammer mill to form the desired uniform size flake or powder final material. Such sequential batch processing method is labor intensive, expensive, and not easily scalable to meet anticipated commercial production demands.